Looking Beyond Silicon

For those of us who have come to assume that the inevitability of silicon chips getting ever smaller and faster was matched only by the inevitability of death and taxes, it is difficult to accept the fact that CPU chip speeds have pretty much hit a plateau in the gigahertz range. There are numerous reasons why, including wildly increased fabrication costs, but the primary villain is that at ever-increasing clock speeds electrons tunneling through smaller, shorter channels result in higher leakage currents and a prodigious, disruptive amount of heat. So if in the future chips are to operate at terahertz frequencies—trillions of operations per second—scientists recognize that a different material will be needed.

The discovery in 2004 of graphene drew excitement among researchers because this material, just one atom thick, possesses exceptional strength and unusual electronic properties, including high electron mobility: electrons travel through graphene more than 100 times as easily as they do through silicon.

Graphene is a sheet of carbon atoms, each atom chemically bonded to its three neighbors to produce a hexagonal array that resembles chicken wire. Unfortunately, the property that makes it a good conductor - its zero energy gap, or band gap, is the key property that makes it very difficult to create graphene transistors, the basic component of logic and memory circuits.

In semiconductors electrons are confined to a number of bands of energy, and forbidden from other regions. The term "band gap" refers to the energy difference between electrons residing in the two most important states of a material − valence band states and conduction band states. The band gap of a material largely determines its electrical and optical properties.

Because graphene does not naturally have a band gap, without some fancy technical tap dancing a graphene device would be always on and hence you can’t do digital logic, since to make any electronic device, like a transistor, you need to be able to turn it on or off.

In 2009, however, researchers based at Lawrence Berkeley National Laboratory engineered a bilayer graphene band gap that can be controlled by an electrical field. This initially encouraging news has in the past three years been tempered by slow progress in making graphene-based electronic devices and recently researchers have been searching for a material that shares some of graphene’s extraordinary properties, but also has this missing quality of a band gap.

Molybdenum disulfide (MoS2), it turns out, just naturally comes with one. About 18 months ago scientists at the Swiss university Ecole Polytechnique Federale de Lausanne’s (EPFL) Laboratory of Nanoscale Electronics and Structures in Switzerland (EPFL suggested that a 2-D layer of MoS2—which occurs as the mineral molybdenite—may serve as a preferable choice over grapheme. They then produced a transistor using MoS2, which isn't a new material by any means, as it's been used for decades as an industrial lubricant and as an element of certain steel alloys (but not in a 2-D form).

Recently researchers at the Massachusetts Institute of Technology (MIT) have succeeded in making a variety of electronic components from a few-atoms-thick MoS2. Their research, which was published in August online in the journal Nano Letters ("Integrated Circuits Based on Bilayer MoS2 Transistors"), described using large sheets of MoS2 (that can be produced using chemical vapor deposition) to fabricate an inverter, a NAND (Negated AND) gate − a basic logic element − a memory device and a ring oscillator made up of 12 interconnected transistors, which can produce a precisely tuned wave output. The circuits comprise between two to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS2. Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.

The paper is authored by Han Wang and Lili Yu, graduate students in MIT’s Department of Electrical Engineering and Computer Science (EECS); Tomás Palacios, the Emmanuel E. Landsman Associate Professor of EECS; graduate student Allen Hsu and MIT affiliate Yumeng Shi, with U.S. Army Research Laboratory researchers Matthew Chin and Madan Dubey, and Lain-Jong Li of Academia Sinica in Taiwan.

The researchers claim the material could help usher in radically new products, from whole walls that glow to clothing with embedded electronics to glasses with built-in display screens.

MoS2 is so thin that it’s completely transparent, and it can be deposited on virtually any other material. For example, MoS2 could be applied to glass, producing displays built into a pair of eyeglasses or the window of a house or office. One of the researchers, Tomás Palacios, believes that the material could find early applications in large-screen displays in which a separate transistor would control each pixel of the display, replacing silicon used in conventional transistors, potentially reducing cost and weight and improving energy efficiency. Also, since one-molecule of thick MoS2 material would be used for the transistors in large-screen displays even a very large display would use only an infinitesimal quantity of the raw materials.

In the future, it could also enable entirely new kinds of devices. Palacios further notes in the MIT press release that the MoS2 when used in combination with other 2-D materials could make light-emitting devices that could be made to make an entire wall glow, making for a warmer and less glaring light that comes from single light bulbs. Similarly, the antenna and other circuitry of a cellphone might be woven into fabric providing a much more sensitive antenna that needs less power and could be incorporated into clothing,

Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semiconductors. According to the researchers, devices with triple MoS2 layers also exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be useful for green light detection.

Molybdenite the principal ore from which molybdenum is extracted is reasonably plentiful and Molybdenum is the 54th most abundant element in the earth's crust and the 25th most abundant element in the oceans. The largest producers are the U.S., China, Chile, Peru and Canada.

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Featured Contributor:
Murray Slovick

Murray Slovick is Editorial Director of Intelligent TechContent, an editorial services company that produces technical articles, white papers and social media posts for clients in the semiconductor/electronic design industry. Trained as an engineer, he has more than 20 years of experience as chief editor of award-winning publications covering various aspects of consumer electronics and semiconductor technology. He previously was Editorial Director at Hearst Business Media where he was responsible for the online and print content of Electronic Products, among other properties in the U.S. and China. He has also served as Executive Editor at CMP’s eeProductCenter and spent a decade as editor-in-chief of the IEEE flagship publication Spectrum.